Irrigation systems exist in the prior art which include automatic control circuits for electrically energized sprinkler valves that turn on the valves when the moisture level in the soil drops below a predetermined value. However, such systems for the most part require separate leads from the sensors to the control circuits, in addition to the leads carrying energizing current to the valves.
The irrigation control system disclosed in Evelyn-Veere et al U.S. Pat. No. 4,176,395 comprises a system in which the moisture sensor signals are sent to a central station over the same leads which carry the energizing power to the valves. However, the system described in this patent uses digitizing techniques and requires relatively complex electrical circuitry at the valves and at the central control station.